The point of the extra x seq is so that consumers that do not evaluate the list element before evaluating the next cons cell don't get such surprising loss of parallelism. For example non-strict left folds like sum will rush to the end of the list spine without evaluating the list elements, and then evaluate the elements later. That does not work well with the parBufferWHNF strategy. Adding the extra seq fixes it. Note that in this case I think seq or pseq would work.